Cannulas are hollow tube instruments used to deliver fluids or remove fluids from blood vessels, ducts or other hollow organs of animals. While many sizes of cannulas are available commercially, microcannulas suitable for use in surgery on small animals are of limited design and utility, The smallest available cannulas are generally flat tipped and are large enough to accommodate a 14 to 24 gauge needle in the lumen of the cannula, which needle is used as a trochar. Small cannulas are generally made of small bore polyethylene or PTFE tubing and are supplied with a hypodermic needle which is used to block a cannula to prevent fluid contained in the vessel of the hollow organ from draining until the cannula is in place in a pre-made incision in the vessel or other hollow organ.
As cannulas decrease in size, they are more flexible and easily bent and therefor difficult to manipulate. The flexibility of small cannulas occurs because of the decreasing absolute wall thickness of the cannula as they get smaller in diameter, and concomitant loss of rigidity of the cannula wall.
Conventional cannulas are supplied with trochars that move freely in the lumen of the cannula since it is conventionally desirable to be able to quickly remove the trochar once a vessel is cannulated.
Conventional small cannulas appropriate for use in cannulation of small blood vessels in microsurgery are notoriously difficult to use. The smallest cannulas available frequently require many minutes of patient and skilled manipulation to prepare a micro-incision in a blood vessel and properly place the cannula in small blood vessels. Furthermore such small cannulas suitable for microsurgery are frequently supported by an outside surrounding heavy gauge needle or auxiliary tube and the microcannula is placed inside the supporting tube. One of the main disadvantages of this conventional arrangement of microcannula and surrounding-supporting tube is that the wound made by the larger supporting hollow tube is larger than that which would be required if the microcannula were more robust.